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{{Page/Header|2.6|Doc:2.6/Manual/Physics/Soft Bodies/Exterior_forces|Doc:2.6/Manual/Physics/Soft_Bodies/Collisions}}

= Interior Forces =

[[File:Bsod-softbody-theory1.jpg|thumb|200px|right|Vertices and forces along their connection edges.]]

To create a connection between the vertices of a Soft Body object there have to be forces that hold the vertices together. These forces are effective along the edges in a mesh, the connections between the vertices. The forces act like a spring. (''Vertices and forces along their connection edges'') illustrates how a 3×3 grid of vertices (a mesh plane in Blender) are connected in a Soft Body simulation.

But two vertices could freely rotate if you don’t create additional edges between them. Have you ever tried building a storage shelf out of 4 planks alone? Well - don’t do it, it will not be stable. The logical method to keep a body from collapsing would be to create additional edges between the vertices. This works pretty well, but would change your mesh topology drastically.
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[[File:Bsod-softbody-theory2.jpg|thumb|200px|right|Additional forces with {{Literal|Stiff Quads}} enabled.]]

Luckily, Blender allows us to define additional ''virtual'' connections. On one hand we can define virtual connections between the diagonal edges of a quad face ({{Literal|Stiff Quads}}, ''Additional forces with {{Literal|Stiff Quads}} enabled''), on the other hand we can define virtual connections between a vertex and any vertices connected to it’s neighbours ({{Literal|Bending Stiffness}}). In other words, the amount of bend that is allowed between a vertex and any other vertex that is separated by two edge connections.
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== Soft Body Edges Settings ==
[[File:Doc-26-Manual-Physics-Softbody-Edges-Panel.png|frame|right|{{Literal|Soft Body Edges}} panel.]]
The characteristics of edges are set with the {{Literal|Soft Body Edge}} properties.

; {{Literal|Anable/Disable Soft Body Edges}}
: Anable/disable using the edges in a Mesh Object to act like springs.

; {{Literal|Springs:}}:
; {{Literal|Pull}}
: The spring stiffness for edges (how much the edges are allowed to stretch). A low value means very weak springs (a very elastic material), a high value is a strong spring (a stiffer material) that resists being pulled apart. 0.5 is latex, 0.9 is like a sweater, 0.999 is a highly-starched napkin or leather. The Soft Body simulation tends to get unstable if you use a value of 0.999, so you should lower this value a bit if that happens.
; {{Literal|Push}}
: How much the Softbody resist being scrunched together, like a compression spring. Low values for fabric, high values for inflated objects and stiff material.
; {{Literal|Damp}}
: The friction for edge springs. High values (max of 50) dampen the {{Literal|Push}}/{{Literal|Pull}} effect and calm down the cloth.
; {{Literal|Plastic}}
: Permanent deformation of the object after a collision. The vertices take a new position without applying the modifier.
; {{Literal|Bending}}
: This option creates virtual connections between a vertex and the vertices connected to it’s neighbors. This includes diagonal edges. Damping also applies to these connections.
; {{Literal|Length}}
: The edges can shrink or been blown up. This value is given in percent, 0 disables this function. 100% means no change, the body keeps 100% of his size.
; ''Vertex Group'' for control {{Literal|Springs}}
: Use a specified vertex group for spring strength values.

; {{Literal|Stiff Quads}}
: For quad faces, the diagonal edges are used as springs. This stops quad faces to collapse completely on collisions (what they would do otherwise).
; {{Literal|Shear}}
: Stiffness of the virtual springs created for quad faces.
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== Preventing Collapse ==
To show the effect of the different edge settings we will use two cubes (blue: only quads, red: only tris) and let them fall without any goal onto a plane (how to set up collision for plane is shown on the page [[Doc:2.6/Manual/Physics/Soft Body/Collisions|Collisions]]).

{|align=right, valign=top
|[[File:Blender3D QuadVSTri-SB-0001-2.49.png|thumb|200px|Frame 1 without {{Literal|Stiff Quads}}.]]
|[[File:Blender3D QuadVSTri-SB-0036-2.49.png|thumb|200px|Frame 36.]]
|[[File:Blender3D QuadVSTri-SB-0401-2.49.png|thumb|200px|Frame 401.]]
|}

In (''Without {{Literal|Stiff Quads}}''), the default settings are used (without {{Literal|Stiff Quads}}). The “quad only” cube will collapse completely, the cube composed of tris keeps it’s shape, though it will deform temporarily because of the forces created during collision.
{{clr}}

{|align=right, valign=top
|[[File:Blender3D QuadVSTri-SB-SQ-0001-2.49.png|thumb|200px|Frame 1 with {{Literal|Stiff Quads}}.]]
|[[File:Blender3D QuadVSTri-SB-SQ-0036-2.49.png|thumb|200px|Frame 36.]]
|[[File:Blender3D QuadVSTri-SB-SQ-0401-2.49.png|thumb|200px|Frame 401.]]
|}

In (''With {{Literal|Stiff Quads}}''), {{Literal|Stiff Quads}} is activated (for both cubes). Both cubes keep their shape, there is no difference for the red cube, because it has no quads anyway.
{{clr}}

{|align=right, valign=top
|[[File:Blender3D QuadVSTri-SB-BS-0001-2.49.png|thumb|200px|Frame 1 with {{Literal|Bending Stiffness}}.<br />[[Media:Blender3D Quads-BE-Stiffness.blend|Blend file]]]]
|[[File:Blender3D QuadVSTri-SB-BS-0036-2.49.png|thumb|200px|Frame 36.]]
|[[File:Blender3D QuadVSTri-SB-BS-0401-2.49.png|thumb|200px|Frame 401.]]
|}

The second method to stop an object from collapsing is to change it’s {{Literal|Bending}} Stiffness. This includes the diagonal edges (Damping also applies to these connections).

In (''With {{Literal|Bending}} Stiffness''), {{Literal|Bending}} is activated with a strength setting of 1. Now both cubes are more rigid.
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{|align=right, valign=top
|[[File:Blender3D QuadVSTri-Bending-001-2.49.png|thumb|200px|Two planes going to collide.]]
|[[File:Blender3D QuadVSTri-Bending-101-2.49.png|thumb|200px|No bending stiffness, Frame 101.]]
|[[File:Blender3D QuadVSTri-Bending-high-101-2.49.png|thumb|200px|High bending stiffness (10), Frame 101.]]
|}

Bending stiffness can also be used if you want to make a subdivided plane more plank like. Without {{Literal|Bending}} Stiffness the faces can freely rotate against each other like hinges (''No bending stiffness''). There would be no change in the simulation if you activated {{Literal|Stiff Quads}}, because the faces are not deformed at all in this example.

Bending stiffness on the other hand prevents the plane from being - well - bent.
{{clr}}

{{Page/Footer|Doc:2.6/Manual/Physics/Soft Bodies/Exterior_forces|Doc:2.6/Manual/Physics/Soft_Bodies/Collisions}}

[[Category:Physics]]
[[Category:Soft bodies]]

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